The electrophotographic development method is a method in which toner particles in a developer are made to adhere to electrostatic latent images formed on a photoreceptor develop the images. The developer used in this method is classified into a two-component developer composed of a toner particle and a carrier particle, and a one-component developer using only a toner particle.
As a development method using the two-component developer composed of a toner particle and a carrier particle among those developers, a cascade method and the like were formerly employed, but a magnetic brush method using a magnet roll is now in the mainstream. In the two-component developer, a carrier particle is a carrier substance which is agitated with a toner particle in a development box filled with the developer to impart a desired charge to the toner particle, and further transports the charged toner particle to a surface of a photoreceptor to form toner images on the photoreceptor. The carrier particle remaining on a development roll to hold a magnet is again returned from the development roll to the development box, mixed and agitated with a fresh toner particle, and used repeatedly in a certain period.
In the two-component developer, unlike a one-component developer, the carrier particle has functions of being mixed and agitated with a toner particle to charge the toner particle and transporting the toner particle to a surface of a photoreceptor, and it has good controllability on designing a developer. Therefore, the two-component developer is suitable for using in a full-color development apparatus requiring a high image quality, a high-speed printing apparatus requiring reliability for maintaining image and durability, and the like. In the two-component developer thus used, it is needed that image characteristics such as image density, fogging, white spots, gradation, and resolving power exhibit predetermined values from the initial stage, and additionally these characteristics do not vary and are stably maintained during the durable printing period (i.e., a long period of time of use). In order to stably maintain these characteristics, characteristics of a carrier particle contained in the two-component developer need to be stable. As a carrier particle forming the two-component developer, various carrier such as an iron powder carrier, a ferrite carrier, a resin-coated ferrite carrier, and a magnetic powder-dispersed resin carrier have conventionally been used.
Recently, networking of offices progresses, and the time changes from a single-function copying machine to a multifunctional machine. In addition, a service system also shifts from a system where a service person who contracts to carry out regular maintenance and to replace a developer or the like to the time of a maintenance-free system. The demand for further extending the life of the developer from the market is increasing more and more.
There are some literatures focusing on such a demand. For example, Patent Literature 1 (JP-A-H08-22150) proposes a ferrite carrier for electrophotographic developer, characterized in that MnO, MgO, and Fe2O3 are partially substituted with SrO, and also describes that according to this ferrite carrier, since variation in magnetization between the ferrite carrier particles is reduced, effects including excellent image quality and durability, environment-friendliness, prolonged lifetime, and excellent environmental stability can be achieved. In addition, Patent Literature 2 (JP-A-2006-17828) proposes a ferrite carrier for electrophotographic developer, characterized by containing from 40 to 500 ppm of zirconium, and also describes that according to this ferrite carrier, occurrence of charge leakage can be prevented due to the high dielectric breakdown voltage thereof, and as a result, high image quality can be obtained.
On the one hand, as such, it has been known that the carrier properties are greatly improved by adding specific additive elements to a ferrite composition, but on the other hand, it has also been known that the carrier characteristics can also be greatly reduced by a trace amount of elements. For example, Patent Literature 3 (JP-A-2011-180296) proposes a carrier core material for electrophotographic developer which is a ferrite core material in which MnO and/or MgO is partially substituted with SrO, characterized in that the Cl concentration of the ferrite core material measured by elution method is from 0.1 to 100 ppm. It is also described that according to this carrier core material, a desired high charge amount can be achieved and an effect of small change in charge amount due to environmental variations can be obtained.
Patent Literature 4 (JP-A-2016-25288) proposes a ferrite magnetic material which includes main components containing Fe and additive elements such as Mn and has an average particle size of from 1 to 100 μm, in which a total amount of impurities excluding Fe, additive elements, and oxygen in the ferrite magnetic material is 0.5 mass % or less, and the impurities include at least two or more of Si, Al, Cr, Cu, P, Cl, Ni, Mo, Zn, Ti, sulfur, Ca, Mn, and Sr. It is described that a magnetic carrier using, as a magnetic carrier core material for electrophotographic developer, the ferrite magnetic material in which the influence of the impurities in the raw material is suppressed, has a high magnetic force and exhibits an effect of suppressing carrier scattering.